LEDs are so easy and cost-effective to use that they are widely applied in various portable apparatus. However, when several LEDs are used for providing a light source, a control circuit is requested for all the LEDs to have uniform brightness. The brightness of a LED is proportional to the driving current flowing through the LED, and therefore the control of the driving current for a LED is the control of the brightness of the LED. Generally, cost is an important concern when providing a current source for driving a LED.
FIG. 1 shows a conventional LED driver 100 comprising several light sources, LEDs 104 and 106, LEDs 108 and 110, LEDs 112 and 114, LEDs 116 and 118, and LEDs 120 and 122, coupled together in parallel, and each light source thereof is coupled in series with a resistor 102. When coupled with a supply voltage VCC, based on the Ohm's law, the LED driver 100 will have the light sources each flowing with a driving current
                              I          n                =                              Vcc            -                          Vd              n                                R                                    [                  EQ          ⁢                      -                    ⁢          1                ]            where n=1, 2, 3, 4, or 5, Vdn is the forward-bias voltage across the respective light source, and R is the resistance of the resistor 12. From the equation EQ-1, the currents I1-I5 supplied for the LEDs 104-122 are dependent on the respective forward-bias voltages Vdn. The forward-bias voltages Vdn varies with the size of the LED, the process to manufacture the LED, and the temperature of the LED in the respective light source, it is therefore hard for the currents I1-I5 to be uniform.
In U.S. Pat. No. 5,025,204 to Su, a current mirror using resistor ratios in CMOS process is provided by using an additional control circuit (i.e., operational amplifiers) to unify the driving currents flowing through several light sources. However, the driving current flowing through a light source is dependent on the battery voltage supplied to the light source, and the additional control circuit will increase the cost. Proposed by U.S. Pat. No. 6,538,394 to Volk et al., current source methods and apparatus for light emitting diodes further employ an adjustable power supply for providing a stable supply voltage to generate driving currents for each of several light sources, in addition to a control circuit to maintain the driving currents of the light sources uniform. This circuit also requires more cost.
Therefore, it is desired a low-cost LED driver.